[diff] Capitalization of headings

[jlaire]

Most clause headings mentioned in Annex C are not capitalized. For example:

C.2.11 Clause 20: general utilities library [diff.cpp03.utilities]
C.3.7 Clause 27: input/output library [diff.cpp11.input.output]

But some of them are.

C.2.16 Clause 27: Input/output library [diff.cpp03.input.output]
C.4.9 Clause 20: General utilities library [diff.cpp14.utilities]

I think they should be all capitalized to match the style in the actual clause headings.

[tkoeppe]

Headings are generally set in sentence case, and our sentence style does not generally capitalize after a colon. So the right solution is to fix the inappropriate capitalization. I'll handle this.